1. Field of the Invention
The invention relates to HID bi-level lighting control systems and particularly to the switching of parallel capacitors in such systems using voltage suppression.
2. Description of the Prior Art
Bi-level dimming systems using high intensity discharge lamps are commonly available in the art, prior systems typically functioning through switching of parallel capacitors on constant wattage (CWA) ballasts. In prior systems, switching has been preferably accomplished through use of electronic zero-crossing switches. Output devices within such systems have typically been back-to-back SCR devices or TRIAC devices. In such a system, a switch must be able to sustain twice the peak voltage of a powered capacitor due to the fact that the switch opens at zero current, zero current being peak voltage for a capacitor. In the following half cycle, the voltage of the powered capacitor opposes that charge remaining on a switched-out capacitor. Although bleeder resistors can be employed to slowly discharge the switched-out capacitor, an appreciable decrease only occurs after a number of cycles. Typical solid state switches are voltage limited in most applications to 400 watts thereby limiting the use of such switches to situations where the RMS capacitor voltages are below 300 volts and where twice peak voltages can exceed 900 volts. Such considerations require switch output devices capable of withstanding 1000 volt breakover. In applications exceeding 400 watts, capacitor RMS voltages can reach 525 volts. A value of twice maximum peak voltage can be approximately 1500 volts. Such design considerations require expensive custom relays that are configurable only at unacceptable cost levels for bi-level dimming systems such as are commonly available.
The prior art is replete with bi-level dimming systems usable with HID lamping as can be seen from a review of issued United States patents. These issued patents, however, do not provide an inexpensive solution to the switching function referred to herein when lamping of high wattage is employed within a bi-level lighting system. As examples, Troy, in U.S. Pat. No. 5,327,048, discloses a bi-level lighting control system for operation of high intensity discharge lamping wherein current surges across electrical magnetic relays are reduced by means of current surge prevention devices. Troy does not provide a system compatible with solid state zero-crossing relays, that is, electronic relays, operable with parallel capacitors. In U.S. Pat. No. 4,931,701, Carl discloses a bi-level control system employing a switch capacitor, the system including a solid state zero-crossing relay. The zero-crossing relay of Carl is disclosed as providing a switching-in or switching-out of a switch capacitor timed to occur at a zero-crossing point of applied voltage. The switch capacitor is thus applied or removed only when voltage level is not available to cause excessive voltage spikes or surges by the switched capacitor if said capacitor is partly or fully charged when switched, thereby eliminating damage to other circuit components. Brosius, in U.S. Pat. No. 6,031,340, discloses a zero current crossing capacity switching arrangement for controlling the switching of a capacitor into and out of an HID lead ballast circuit at a time when a current through the capacitor is at or near zero, thereby enabling bi-level operation of an HID lamp. In U.S. Pat. No. 5,811,939, Herniak discloses a bi-level control system utilizing a programmable logic controller for controlling switching in a predetermined sequence to allow switching between voltage sources. Kahn et al, in U.S. Pat. No. 5,451,843, discloses circuitry for providing bi-level illumination utilizing a “random crossing” relay. Guidette et al, in U.S. Pat. Nos. 5,227,762 and 5,475,360 describe a lighting system controllable through a power line carrier system. Nuckolls et al, in U.S. Pat. No. 6,114,816, describes a lighting control system for discharge lamping for switching said lamping between dimmed and normal wattage operational modes.
The bi-level lighting control systems of the prior art including such systems as are described in the aforesaid United States patents do not accommodate the use of high wattage lamping in such systems with low voltage rated switching devices. A system permitting the use of high wattage tamping in bi-level HID lighting control systems at cost levels associated with market requirements in the industry therefore provides a substantial advance in the art.